Number of known followers:
Lord Voldemort: 28 (Gaspar Avery, Regulus Arcturus Black (D), Alecto Carrow, Amycus Carrow, Crabbe Sr, Barty Crouch Jr. (D), Antonin Dolohov, Gibbon (D), Goyle Sr., Jugson, Igor Karkaroff (D), Bellatrix Lestrange (D), Rodolphus Lestrange, Rabastan Lestrange, Walden Macnair, Draco Malfoy, Lucius Malfoy, Mulciber, Nott Sr., Peter Pettigrew (D), Augustus Rookwood, Evan Rosier (D), Thorfinn Rowle, Selwyn, Severus Snape (D), Travers, Wilkes (D), Corban Yaxley)
Grindelwald (As of FB 3):10 (Abernathy, Carrow, Krafft, Krall (D), MacDuff, Nagel, Vinda Rosier, Credence Barebone / Aurelius Dumbledore, Queenie Goldstein, Yusuf Kama)
Name (D): denotes that they died, as seen in the material available at the current date.
Voldemort 1 : 0 Grindelwald.
How did they gain followers?
Lord Voldemort: Lying, Fear.
Grindelwald: Lying, Charisma.
Voldemort 1 : 1 Grindelwald.
Number of followers who defected / were unloyal / spied on them.
Lord Voldemort: 2/3 (Regulus Black, Severus Snape, possibly Draco Malfoy).
Grindelwald (As of FB 3):4 (Krall, Yusuf Kama, Queenie Goldstein, Credence Barebone / Aurelias Dumbledore).
Voldemort 2 : 1 Grindelwald.
How many were killed because of their disloyalty?
Lord Voldemort: 1? (Regulus. Depends on whether you consider dying by security measures as being killed for disloyalty. Snape died due to a case of mistaken identity of ownership of the Elder Wand).
Grindelwald (As of FB 3): 1 (Krall died in the blue flames spell in the Parisian catacombs).
Voldemort 2 : 2 Grindelwald.
Killed by:
Lord Voldemort: 17 year old wizard, Harry Potter.
Grindelwald: 72 year old Dark Lord, Voldemort.
Voldemort 2 : 3 Grindelwald.
Worst odds they’ve faced in a battle (with or without followers helping) (number of opponents : them + number of followers):
Lord Voldemort: 3:1 (Battle of Hogwarts, Voldemort vs McGonagall, Slughorn and Shacklebolt)
Grindelwald (As of FB 3): 53:1 (Parisian catacombs, Grindelwald vs 50 Aurors, Newt, Theseus and Tina)
Voldemort 2 : 4 Grindelwald.
From the above battle, how many opponents did they kill / knockout?
Lord Voldemort: 0.
Grindelwald: At least 11 (Leta and 10 Aurors, from what I could make out from rewatching the scene over and over).
Voldemort 2 : 5 Grindelwald.
Best odds they’ve faced in a battle (with or without followers helping) (number of opponents : them + number of followers):
Lord Voldemort: 1: at least 7 (GoF, his rebirth, Harry vs Voldemort and at least 6 Death Eaters).
Grindelwald (As of FB 3): 5:1 (1st FB movie, opening scene, Grindelwald vs 5 Aurors).
Voldemort 2 : 6 Grindelwald.
From the above battle, how many opponents did they kill / knockout?
Lord Voldemort: 0
Grindelwald: 5 (The scene shows them falling to the ground after being hit by a spell)
Voldemort 2 : 7 Grindelwald.
How do they stop Albus fighting them? (By fight, I mean fight to kill)
Lord Voldemort: Albus believed someone else would kill Voldemort.
Grindelwald: They made a Blood Pact, though it only worked until the Blood Pact was destroyed.
Voldemort 2 : 8 Grindelwald.
How do they summon their followers?
Lord Voldemort: Dark Mark on the left forearm (discreet, permanent, makes it obvious who’s a follower if someone sees it).
Grindelwald (As of FB 2): Black cloth over the buildings of Paris (showy, disappears when Grindelwald wants it gone, no mark on followers).
Voldemort 2 : 9 Grindelwald.
They were both looking for a great weapon. What was it?
Lord Voldemort: Prophecy / Elder wand.
Grindelwald (As of FB 2): Obscurus.
Voldemort 2 : 10 Grindelwald.
Age when they died:
Lord Voldemort: 72
Grindelwald: 115.
Voldemort 2 : 11 Grindelwald.
Did they go to prison?
Lord Voldemort: No.
Grindelwald: Yes.
Voldemort 3 : 11 Grindelwald.
How many of their followers went to prison?
Lord Voldemort: At least 4 (Bellatrix, Rodolphius, Rabastan, Barty Crouch Jr).
Grindelwald (As of FB 3): 0.
Voldemort 3 : 12 Grindelwald.
Highlight of their career:
Lord Voldemort: Taking over the Ministry.
Grindelwald:Becoming Supreme Mugwump (for a hot minute).
Voldemort 3 : 13 Grindelwald.
Most showy thing they've done:
Lord Voldemort: 1996, duel between him and Albus in the Ministry.
Grindelwald (As of FB 2): 1927, battle in the Parisian catacombs.
Voldemort 3 : 14 Grindelwald.